There is currently an increasing demand for water soluble, biodegradable polymers to replace the substantial amount of non-biodegradable polymers that are in the marketplace. Non-biodegradable polymers place a significant demand on resource due to the requirement for their disposal in landfill sites or by incineration.
Poly vinyl alcohol (PVA) is recognized as one of the very few vinyl polymers soluble in water that is also susceptible of ultimate biodegradation in the presence of suitably acclimated microorganisms. Accordingly, increasing attention is being devoted to the preparation of environmentally compatible PVA-based materials for a wide range of applications. PVA has excellent film and thin-walled container forming properties, demonstrating a high degree of impermeability to a number of gases, making it highly suitable for use in packaging products for release in an aqueous environment. It also has high adhesive strength, and is nontoxic. However, these properties are dependent on humidity due to the polymer absorbing water which reduces its tensile strength but increases its elongation and tear strength. It is also difficult to successfully extrude PVA or PVA-containing compositions which further limits its potential use. In particular, there are currently no formulations that can be readily moulded to a minimum thickness required (less than 200 microns) to ensure product release at low aqueous temperatures, typically at or below 5° C. in less than 2 minutes. Such a film is desirable for applications such as packaging of laundry products which require release of the laundry detergent at low temperatures and on a short wash cycle.
One of the properties of the polymer that it is desirable to improve is the Melt Flow Index. This relates to the ease of flow of the melt of the polymer, defined as the mass of the polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. One such method is described in the standard ISO 1133. A higher MFI is required to provide a thin walled moulding.
It is known in the art to use internal lubricants within the PVA to increase its melt flow index. For example, EP1112316 B1 (PVAXX Technologies Limited) includes a fatty acid amide in an amount up to 5% by weight. The fatty acid provides lubrication between the polymer chains thereby increasing the melt flow of the polymer. However, they are insoluble, melting and coating the polymer during processing, which may impede dissolution of the polymer. Furthermore, the amount of lubricant that can be used is limited due to excess lubricant (generally above 5% by weight of the PVA) separating out of the blend, thereby restricting their ability to improve the MFI of a polymer.
The polymer is produced by polymerisation from vinyl acetate and subsequent hydrolysis of the polyvinyl acetate (PVAc) which is formed. PVA and PVA-derived polymers are soluble in water with their solubility being determined by the molecular weight of the polymer and the degree of hydrolysis, i.e. the percentage of acetate groups of the starting polymer (PVAc) that has been replaced with OH groups. The higher the degree, the lower the solubility and the speed of solution. The differences are much more marked at low dissolving temperatures than at high ones due to the formation of crystalline zones within the polymer.
It is also known in the art that PVA requires the removal of volatiles from the composition before melt processing as without such removal processing is difficult due to the formation of steam and subsequent foaming of the polymer. Drying is generally achieved using standard drying equipment at a temperature of 90° C. for a period of 4-9 hours, depending on make, model and formulation.
Despite attempts to mould articles from PVA or PVA derived compositions, the desired solubility characteristics have not been achieved due to the lack of melt flow required to mould a thin walled article that will dissolve in an aqueous solution within a desired time frame.
It is an aim of the present invention to provide water soluble polymer compositions, particularly but not exclusively polyvinyl alcohol compositions, that address or at least alleviate the aforementioned problems experienced with the polymer compositions of the prior art.
Further aims of the present invention are to provide methods for the production and extrusion and/or moulding of water soluble polymer compositions.